1. Field of the Invention
This invention relates to a digital demodulating apparatus for demodulating, for example, a received narrow band modulated signal using digital signal processing.
2. Description of the Prior Art
A demodulating apparatus is employed in a radio wave receiver which receives a narrow band signal such as a mobile radio telephone set or such as an automobile telephone set. Such a demodulating apparatus is often constructed so as to conduct digital signal processing, and in such construction, it performs so-called quasi-synchronous detection in order to effect analog to digital conversion.
FIG. 6 shows an exemplary conventional demodulating apparatus which performs such quasi-synchronous detection. Referring to FIG. 6, the demodulating apparatus shown is constructed so as to receive and demodulate a signal of the PSK/TDMA communications system and includes a band pass filter 11 to which a received M-phase PSK signal is supplied. The band pass filter 11 thus removes out-band components, that is, unnecessary frequency band components other than an object frequency band component from the received M-phase PSK signal. An output signal r(t) of the band pass filter 11 is represented by the following expression: ##EQU1## where A is a level of a magnitude, fc' is a frequency of a carrier, .phi.k is information of a phase of a k-th symbol, .theta.r is an initial phase, T is a period of symbols, and n(t) is a noise, and (k-1)T.ltoreq.t.ltoreq.kT.
Such an output signal of the band pass filter 11 is supplied to a pair of mixer circuits 12C and 12S.
The demodulating apparatus further includes a fixed oscillator 13 for generating a reference signal Rc(t) having a fixed frequency fc in the proximity of the carrier frequency fc' and an initial phase .theta..sub.R. An output signal of the fixed oscillator 13 is supplied to the mixer circuit 12C and also to a .pi./2 phase shifting circuit 14, by which the phase of the signal Rc(t) from the fixed oscillator 13 is shifted by .pi./2 to obtain a reference signal Rs(t). The reference signal Rs(t) thus obtained is supplied to the other mixer circuit 12S. Consequently, an output signal of the band pass filter 11 is quasi-synchronously detected with such two reference signals Rc(t) and Rs(t) which have an orthogonal relationship in phase to each other.
Output signals of the mixer circuits 12C and 12S are supplied to a pair of low-pass filters 15C and 15S, respectively, at which high frequency components are removed from the signals, and then the signals are supplied to a pair of analog to digital (A/D) converters 16C and 16S, respectively, at which the signals are converted into digital signals. Here, the sampling frequency at the analog to digital converters 16C and 16S is defined as f&gt;2B.sub.1 where B.sub.1 is a band pass width of the band pass filter 11 so that it may satisfy a sampling theorem.
Digital data of two quasi-synchronous detection signals having an orthogonal relationship in phase to each other which are obtained from the analog to digital converters 16C and 16S in this manner are represented in the following manner in complex number representation; EQU x=x(t)=Ae.sup.-j(2.pi.fet+.phi.k+.theta.e) +n'(t) (2)
where fe is a difference between the fixed frequency fc of the reference signal and the carrier frequency fc' of a received signal, and .theta.e is a difference in initial phase between the reference signal and the received signal. If the parameters fe and .theta.e can be presumed, then two reference signals having orthogonal phases to each other and synchronized with the received signal can be reproduced. If such two signals having an orthogonal phase relationship to each other are obtained, then a magnitude and a phase of the input signal can be demodulated from the two signals.
Digital data from the analog to digital converters 16C and 16S are supplied once to a buffer memory 17 and then to a demodulator 18 of the digital signal processing type. At the demodulator 18, frequency and phase as described above is presumed to conduct demodulation of the two input signals.
The conventional demodulating apparatus described above, however, is disadvantageous in that it requires two sine and cosine components as reference signals, and besides the orthogonality of such two components must be adjusted with a very high degree of accuracy so that the error thereof may be lower than 3 degrees, or else, a required performance cannot be achieved.
Furthermore, since a system including a mixer circuit, a low pass filter and an analog to digital converter is required for each of two components having an orthogonal phase relationship to each other, a total of two circuits are required. In addition, since each of the output signals of the low-pass filters 15C and 15S is a base band component which includes a dc current component therein, an amplifier for such base band component including a dc current component is required, which makes construction of the demodulating apparatus difficult.